WASHINGTON — The Obama administration said Monday that it had “undeniable” evidence that the Syrian regime of Bashar Assad used chemical weapons to kill hundreds of civilians, significantly raising the possibility that the United States would take military action in the conflict.

Secretary of State John F. Kerry, in the government’s toughest public stance yet on last week’s attack, said Syrian forces used nerve agents against men, women, and children in a rebel-held neighborhood. He appealed to other countries to join the United States in taking action for what he called a “moral obscenity.”

“Make no mistake: President Obama believes there must be accountability for those who would use the world’s most heinous weapons against the world’s most vulnerable people,” Kerry said in a brief appearance at the State Department, insisting that “all peoples in all nations who believe in the cause of our common humanity must stand up to assure there is accountability for the use of chemical weapons so that it never happens again.”

But it was not immediately clear whether other nations would join the United States in a possible strike against Syria. Russia has said there is no evidence that chemical weapons have been used and would probably block an effort at the United Nations for a resolution demanding action. That could put the Obama administration in the position of deciding whether to act unilaterally or with a small coalition.

Obama has said that using chemical weapons would cross a “red line,” suggesting that it would require a military response. Syria has denied involvement in the attack even as reports indicate it has tried to destroy evidence.

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“President Obama believes there must be accountability for those who would use the world’s most heinous weapons,” Secretary of State John Kerry said.

Kerry’s harsh words were delivered hours after UN inspectors trying to reach the site of the attack were fired upon.

The prospect of even a limited air or missile campaign has fueled fears among some that the United States could be dragged into another quagmire shortly after ending its involvement in the Iraq war and as it prepares to withdraw forces from Afghanistan. As a result, much of the discussion Monday was about limited action and surgical strikes, which in turn prompted analysts to question whether that would be enough.

‘President Obama believes there must be accountability for those who would use the world’s most heinous weapons.’

Leading Republicans and Democrats called for a US military response.

Senator Edward J. Markey, a Massachusetts Democrat and member of the Foreign Relations Committee, said he would support “a surgical set of strikes” if they were made against the sites that could be used to launch the chemical weapons.

“I think it is important for the United States to make this statement and to be a leader on this issue,” he said in an interview.

Markey also called on the White House to seek approval for any military action from Congress.

Some leading humanitarians who are commonly wary of military engagements saw few other options after Syria’s apparent use of chemical weapons.

“I think it is absolutely clear that President Obama will respond militarily,” Kenneth Roth, executive director of Human Rights Watch, said in an interview. “His credibility is at stake. This is a horrible weapon that should never be used.”

Roth said the last attack of such scale appeared to be the use of poison gas in the town of Halabja in Iraq in 1988 by the regime of Saddam Hussein.

Navy officials confirmed Monday that four destroyers have been dispatched to the eastern Mediterranean. The destroyers Gravely, Mahan, Barry, and Ramage are each outfitted with long-range Tomahawk cruise missiles and artillery guns that could strike Syrian military targets.

The actions came amid debate about whether any US-led military action should be a limited strike to punish the regime and deter future use of chemical weapons, or a broader effort aimed at toppling Assad, who has been battling rebel groups with varying agendas, including those with ties to Al Qaeda terrorists.

Among those calling for a stronger air campaign was Senator John McCain, an Arizona Republican, who on Sunday said any US military intervention should be intended to remove Assad from power and end the civil war.

“We can significantly degrade Assad’s air power and ballistic missile capabilities and help to establish and defend safe areas on the ground,” McCain said in a joint statement with Senator Lindsey Graham, a South Carolina Republican.

Others agreed that pinpoint strikes probably would not affect the outcome of the conflict.

“I am not sure Tomahawk missile strikes are going to make a difference,” said Nawaf Obaid, a research fellow at the Belfer Center for Science and International Affairs at the Harvard Kennedy School. “It’s too little too late. There needs to be a strategy that is much more robust.”

But others have warned against embroiling the United States in another war.

The nation’s top military officer, General Martin Dempsey, has urged caution, warning Congress that any US military involvement could empower the Sunni extremists that intelligence officials believe have become a major player in the anti-Assad opposition.

Senator Jack Reed, a Rhode Island Democrat and member of the Armed Services Committee, echoed those concerns Monday.

“We have to be very careful to define what our objective is,” he said in an interview. “Our objective is primarily to dissuade permanently the Syrians or any other power from using chemical weapons or other weapons of mass destruction. That needs to be the focus.”

Recent polls suggest that Americans have deep reservations as well. A Reuters/Ipsos poll released Monday indicated that nearly 60 percent of Americans believe the United States should stay out of Syria’s civil war, even if chemical weapons are used.

Even some hawkish voices questioned whether a robust air campaign would achieve US goals.

“There is no way the US can quickly use any amount of force to destroy the Assad regime with any confidence that Syria will not come under Sunni Islamist extremist control, or divide into Alawite, Sunni, and Kurdish blocs in ways that prove to be even more violent and lasting than such sectarian and ethnic divisions have in Iraq,” Anthony Cordesman, a specialist at the Center for Strategic and International Studies, wrote in a new paper Monday.

Secretary of Defense Chuck R. Hagel said any US military action would require international support.

“We are analyzing the intelligence and we will get the facts and if there is any action taken it will be in concert with the international community and within the framework of a legal justification,” Hagel told reporters in Jakarta, Indonesia.

But the toughest talk came from the nation’s top diplomat, Kerry, who spent the weekend conferring with US allies.

“What we saw in Syria last week should shock the conscience of the world,” he said Monday. “The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity,”

Kerry said a decision on how to respond would be made in the coming days.

“The administration is actively consulting with members of Congress and we will continue to have these conversations in the days ahead,” he said. “President Obama has also been in close touch with the leaders of our key allies, and the president will be making an informed decision about how to respond to this indiscriminate use of chemical weapons.”